The present invention relates to systems for inspecting tubular products, especially products which are subject to tube wall degradation and thinning. In particular, the present invention relates to a system for inspecting the thinned region of tube walls and for determining the extent of "wastage", i.e., the volume of material lost. While the invention may be useful with any type of tubular products, it has particular application to the inspection of the heat exchange tubes of a nuclear steam generator.
A nuclear steam generator contains many vertical tubes aligned in rows and columns in close relationship to each other, and having their lower ends mounted in a tube sheet. A primary fluid, having been heated by circulation through the nuclear reactor core, is circulated through the tubes. At the same time a secondary fluid, known as feedwater, is circulated around the tubes in heat transfer relationship therewith, thereby transferring heat from the primary fluid in the tubes to the secondary fluid surrounding the tubes, causing a portion of the secondary fluid to be converted to steam.
Deposits tend to settle out of the feedwater and build up along the tube sheet forming deposits known as "sludge". This sludge alters the chemistry of the feedwater, causing it to attack the outer surfaces of the tubes, resulting in localized pitting, corrosion loss or cracking. The integrity of the tubes can also be degraded by mechanical stresses which may result in wear scars or the like.
Isolation of the radioactive primary fluid from the secondary fluid in a nuclear steam generator is critical. Accordingly, test procedures have been developed to test the integrity of the generator tubes. One such procedure is an inspection process, wherein a defect-sensing probe is inserted into a tube and retracted from it at a constant rate. In the event that defects are located in the tube, such defects may be corrected by sleeving, i.e., mounting an auxiliary tube inside the defective tube to span the defective region, thereby returning the tube to its normal heat transfer capacity.
Typically, this testing of the tube integrity is accomplished using eddy current techniques, wherein a probe sets up an electromagnetic field which induces eddy currents in the tube wall, which are in turn detected by a sensor in the probe. The nature of the eddy currents is affected by discontinuities in the tube wall. While the eddy current technique is successful for the detection and characterization of many types of tubing degradation, such as cracks, pits, wear, etc., these techniques have not proved reliable for the detection of discontinuities with dimensions less than about 10% of the tubing wall thickness. Consequently, when it is necessary to develop a precision characterization of tubing damage to assess projected performance, eddy current methods are not always adequate. For example, the accurate assessment of tubing erosion or wear, i.e., wall thinning, requires data concerning the volume of material removed from the tube wall. This volume may be significant even though the wear scar is very shallow, if its overall area is sufficiently large. Yet eddy current techniques cannot accurately detect and characterize such shallow wear scars.